Most of the present-day gas sensors based on carbon nanotubes are conductometric, that is, a response of the sensor based on the resistance change due to contact with the gas. That change is usually due to a doping process.
The electrochemical sensors which are popular fail in sensing oxygen in aircraft engines, due to the lack of accuracy and contaminants from interfering fuel vapors. The “wet” electrochemical sensor has inherent disadvantages of leakage and the “dry” electrochemical sensors with solid electrolytes (e.g., cationic and anionic membranes, xerogels) have lesser sensitivity and are influenced by humidity and temperature. There is a need for an inexpensive sensor that can operate in high temperature, pressure conditions, that can be inert towards fuel gas vapors and they can have increased accuracy towards the detection of oxygen in fuel tanks.
As a result of the foregoing, there is a need in the art for a sensor to monitor oxygen content. The sensor should be unaffected should it come in contact with either fuel vapors or the fuel itself. The explosive limit of oxygen such as in aircraft fuel tanks is between 9-12%, however an efficient sensor should be able to detect <9% and >12% of oxygen in the aircraft fuel tanks. The sensor should have low maintenance, high reliability, low cost and reasonable recovery times. The normal electrochemical sensors attractive for the detection of various gases are not suitable for this application, due to the possible contamination of the electrolyte with the fuel vapors.
Other needs for related gas sensors occur in automotive and commercial combustion control systems, where the amounts of residual oxygen in the effluent gasses must be measured. In other applications, there is a need to detect chemical warfare (nerve) agents to counter terrorism. The chemical warfare agents are mostly nucleophiles (electron donors) and need to be detected in ppb (parts per billion) concentrations. The accurate detection of these chemical species is essential for the safety of human life in protecting the general public from possible terrorist attacks using chemical warfare agents.
Present-day conductometric sensors cannot distinguish between different electron donating or electron accepting species. This leads to cross-sensitivity or interference between the different gaseous species. In view of theses findings, there is a need in the art for a highly selective sensor for the detection of electrophile (e.g., oxygen and other electron acceptors) and nucleophile (e.g., chemical warfare agents and other electron donors) gases. Furthermore, a sensor using a diode characteristic as the electronic detection mechanism, instead of the standard conductometric mechanism, could provide higher sensitivity and lower detection limits.